Breeder reactor

A reactor that makes more fissile fuel from fertile material, usually uranium-238 or thorium-232, than it uses up, so its stock of fissile material grows. The measure is the conversion ratio, new fissile atoms made for each fissile atom consumed: above 1 is breeding, about 1 is break-even, and below 1 is a converter, or a burner if well below. Ordinary light-water reactors make some plutonium from uranium-238 but have conversion ratios well below 1. Fast reactors are the usual route to breeding with uranium and plutonium, because at high neutron energies plutonium-239 releases more new neutrons for each neutron it absorbs, leaving more spare to convert uranium-238. Thorium is the exception that can breed with slow neutrons, narrowly: the Shippingport light-water breeder core in the United States, run from 1977 to 1982, ended with about 1.4% more fissile material than it started with. Whether a fast reactor breeds or burns is largely a design and fuel-management choice, so 'capable of breeding' does not mean a reactor is breeding, and few reactors have ever run as breeders commercially.

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concept:breeder-reactor

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